The present invention relates to a filter circuit and a superconducting filter circuit each for limiting bandwidth of a transmitter such as a portable wireless terminal or a base station using wireless communication.
As shown in FIG. 1, a filter circuit for bandwidth limit has cascaded resonators 153a, 153b, and 153c cascaded. In FIG. 1, an equivalent circuit of each resonator includes an inductor and a capacitor. If loss effect is also taken into consideration, then resistance is added.
Assuming non-resistance, the resonance frequency f0 of the resonator is represented as follows.
f0=1/{2xcfx80{square root over ( )}(Lxc2x7C)}
(L: inductance of the resonator, C: capacitance of the resonator)
In the filter circuit, a plurality of resonators are cascaded. Accordingly, by adequately setting the coupling factor between resonators xe2x80x9cM2, M3,xe2x80x9d representing a coupling factor of each resonator and external Q (coupling factor between resonators xe2x80x9cM1, M4xe2x80x9d) representing a value to excite the resonator by input/output unit, designable frequency band and attenuation of stop-band for the filter circuit are determined.
As one example of the filter circuit, FIG. 2 shows a component example using microstrip lines. In FIG. 2, microstrip conductors 161, 162, 163a, 163b, and 163c are formed on the surface of a dielectric substrate 164. On the reverse side of the dielectric substrate 164, a ground metal (not shown in FIG. 2) is set. The microstrip conductor, the dielectric substrate, and the ground metal compose the microstrip line. In order to compose three resonators, the microstrip conductors 161 and 162 are formed on a main face of the dielectric substrate. Three microstrip conductors 163a, 163b, 163c having length equal to the half-wave length of the designable frequency band are formed between excitation lines 161, 162 of the input/output side by shifting every xc2xc wave length. For example, the component element of the resonator is the microstrip conductor 163a and a circumferential dielectric (the dielectric substrate 164 and an exposure part (air)). The space between the resonators determines the value of the coupling factor between the resonators. The excitation lines 161, 162 of input/output side are located at a distance representing a desirable external Q from the resonator.
In many filter circuits, all resonators are cascaded. As a result, electric power passing through the filter circuit passes each resonator by the same electric power. However, the resonator includes loss effect, and pass-electric power is slightly different because of the loss effect. The filter circuit through which large electric power passes is heated by the loss effect, and it is important that the filter circuit includes a radiating thermal component. In case that a distributed element circuit such as the microstrip line is used as the filter circuit, the circuit component becomes small. However, in this case, the loss becomes large and the radiating thermal characteristic falls.
Accordingly, in order to realize a low loss and a small circuit size, the microstrip line filter circuit in which a superconducting conductor is used as the microstrip conductor is utilized. In this case, lines of electric force are generated in this microstrip line. As shown in FIG. 3, the lines of electric force 174 generates in the dielectric 173 between the microstrip conductor 171 and the ground metal 172, and an electric field concentrates on a sectional edge of the line (the microstrip conductor 171) through which signal electric power passes. In short, electric current concentrates on this edge part. Accordingly, in case of passing a large electric power, the electric current flowing through the edge exceeds the threshold of critical current density in spite of several-watt electric power, and it breaks the superconducting characteristic.
As mentioned-above, in known filter circuits, a plurality of resonators are cascaded in order to vary the designable frequency band. However, when the large electric power passes through the filter circuit of cascade connection, the large electric power equally passes through all resonators in the filter circuit. Accordingly, large characteristic of maximum available power is necessary for this filter circuit.
Furthermore, in the filter circuit using the microstrip line resonator, in case of passing the large electric power, the electric current concentrates on the edge of the microstrip conductor. Accordingly, in case of using the superconducting conductor, the electric current exceeds the critical current density and breaks the superconducting characteristic.
It is an object of the present invention to provide a filter circuit and a superconducting filter circuit of small size having superior characteristic of maximum available power.
According to the present invention, there is provided a filter circuit, comprising: a first resonator and a second resonator each having a different resonance frequency, a first block including the first resonator and a second block including the second resonator, wherein the first block includes a first delay unit connected to the first resonator; an input terminal configured to divide an input signal to the first block and the second block; and an output terminal configured to combine signals passing through the first block and the second block, and to output the combined signal; wherein said first delay unit converts a phase difference between the signals passing through the first block and the second block to reverse-phase or nearly reverse-phase.
Further in accordance with the present invention, there is also provided a superconducting filter circuit, comprising: a first resonator and a second resonator each having a different resonance frequency, a first block including the first resonator having a superconcuctive material and a second block including the second resonator having a superconductive material, wherein the first block includes a delay unit connected to the first resonator; an input terminal configured to divide an input signal to the first block and the second block; and an output terminal configured to combine signals passing through the first block and the second block, and to output the combined signal; wherein said delay unit converts a phase difference between the signals passing through the first block and the second block to reverse-phase or nearly reverse-phase.
Further in accordance with the present invention, there is also provided a filter circuit, comprising: a first resonator and a second resonator each having a variable resonance frequency, a first block including the first resonator and a second block including the second resonator, wherein the first block includes a delay unit connected to the first resonator; an input terminal configured to divide an input signal to the first block and the second block; and an output terminal configured to combine signals passing through the first block and the second block, and to output the combined signal; wherein the resonance frequency of at least one of the first resonator and the second resonator is varied by an external control signal.
Further in accordance with the present invention, there is also provided a superconducting filter circuit, comprising: a first resonator and a second resonator each having a variable resonance frequency, a first block including the first resonator and a second block including the second resonator, wherein the first block includes a delay unit connected to the first resonator; an input terminal configured to divide an input signal to the first block and the second block; and an output terminal configured to combine signals passing through the first block and the second block, and to output the combined signal; wherein the resonance frequency of at least one of the first resonator and the second resonator is varied by control signal from external.